Elevator door monitors are devices that enhance the functionality and safety of elevator control systems. Conventional door monitoring systems rely on customization of the input stage, so that they can be compatible with different types of control systems. The most common device for interfacing with an elevator controller is a relay. A relay is an electromechanical switch that activates only if: (i) an input voltage applied to the relay equals or exceeds a built-in threshold voltage value, and (ii) the current entering the relay is of the type (AC or DC) at which the relay is configured to operate. Input relays are used for translating voltage at different levels into a normalized voltage that can be used by an elevator controller. Each monitoring device within conventional systems monitors the door(s) of a single elevator car. Each such device monitors several different types of signals to accomplish that task, including but not limited to Door Fully Closed (DFC), Door Fully Open (DFO), Door Gate Switch (DGS), Hoistway Door Lock (HDL), and Fireman Service 2 (FS2). Furthermore, many conventional input devices or modules (including but not limited to relay-based devices) use at least one relay per each signal that needs to be monitored in order to address variability in voltage level and types. When one of the signals that needs to be monitored changes state, the elevator controller transmits a voltage via a current (again, of type either AC or DC) to the input device. If that preset voltage equals or exceeds the built-in threshold voltage of that input device (and the current types match), the input device activates and thereby communicates the sensed state of that particular signal to the logic block of the door monitor. If the voltage entering the relay is below the built-in threshold voltage value, the input device remains inactive. Thus, successful operation of a conventional monitoring system requires that the built-in threshold voltage value of the input device match the preset change-of-state voltage of the elevator car, and that the current types are the same for both the elevator and the relay.
There is a lack of uniformity in the design of elevator controllers, such that one elevator of a building frequently generates a change-of-state voltage at a different magnitude than that of another elevator, and also possibly via differing current types. This can especially be the case if some elevator controllers are part of “legacy” systems that were included in the original construction of the building, and some were added later in the life of the building. Thus, each building may have different types of elevator controllers, each different elevator controller generating a different magnitude of a change-of-state voltage for a given event. For this reason, an elevator door monitoring system must frequently be custom-built so that its input modules or devices correspond with the elevators at matching built-in voltages and current types.
The use of relay switches and other input devices in conventional elevator door monitoring system presents several drawbacks. Having to custom-build each door monitoring system, as described above, is expensive and time-consuming. Additionally, the mechanical parts in the relays are vulnerable to failure that limit the life expectancy of the relay. Still further, relays are noisy when in operation. Furthermore, a system with a multitude of relays presents room for improvement, both in terms of power consumption and in physical size of the elevator door monitoring system.